Abstract
In A7r5 vascular smooth muscle cells, Arg(8)-vasopressin (AVP) stimulates phospholipase C leading to activation of two distinct Ca(2+) entry pathways. The capacitative Ca(2+) entry (CCE) pathway is activated by depletion of Ca(2+) stores, is permeable to Mn(2+), Ba(2+) and Ca(2+), and is selectively blocked by Gd(3+)(1 microM). A7r5 cells also express a non-capacitative Ca(2+) entry (NCCE) pathway, which is activated by arachidonic acid that is released by the sequential activities of phospholipase C and diacylglycerol lipase. This pathway is permeable to Sr(2+), Ba(2+) and Ca(2+) and selectively blocked by (R,S)-(3,4-dihydro-6,7-dimethoxy-isochinolin-1-yl)-2-phenyl-N,N-di[2-(2,3,4-trimethoxyphenyl)ethyl]acetamid mesylate ("LOE-908"). We use these selective tools to show that AVP, via the same signalling pathway that leads to activation of NCCE, also inhibits CCE and that the inhibition is not due to depolarization of the plasma membrane. Using the selective inhibitors to resolve the contributions of each Ca(2+) entry pathway during stimulation with AVP, we establish that reciprocal regulation of CCE and NCCE by arachidonic acid ensures that only NCCE is active in the presence of AVP, whereas CCE is active only after its removal. NCCE and CCE are therefore activated in a strict temporal sequence: NCCE first and then CCE. Because Ca(2+) passing through different Ca(2+) entry pathways can selectively regulate different responses, reciprocal regulation of CCE and NCCE may allow a stimulus to first evoke a response and then recruit actively a different response when the stimulus is removed.